In a typical nuclear reactor power plant, nuclear fission is used to generate heat for the production of steam and electricity. In one such power plant, a pressurized water reactor for example, fissile material, generally in the form of uranium 238 enriched by uranium 235, is bombarded by neutrons to cause a fissionable reaction. The fission process produces additional neutrons which sustain the fissionable reaction within the reactor vessel. In such a vessel, nuclear fuel is arranged in a generally rectangular shaped reactor core within a generally cylindrical pressure vessel. Between the closely packed nuclear fuel, in the form of elongated fuel rods mounted within square shaped fuel assemblies, and the reactor vessel is an assembly commonly referred to as the reactor internals. The reactor internals typically comprise an upper core support plate, a lower core support plate and a core barrel. The core barrel is an elongated, generally cylindrically shaped structure radially situated between the reactor core and the cylindrical wall of the pressure vessel.
In the irregular space between the core and the core barrel, commonly referred to as the baffle-barrel region are structural components. (See FIG. 1.) Baffle plates which extend the length of the core are supported at several axially spaced radial locations by formers which, in turn, are attached to the core barrel. Radially outward beyond the core barrel is a thermal shield, a downcomer annulus, and then the reactor vessel. The baffle plates, formers, core barrel, and thermal shield are typically made of stainless steel; the downcomer annulus and most of the region between the baffle plates and the core barrel is mostly occupied by coolant.
To remove the heat generated within the core, the reactor coolant, typically borated water, circulates through the pressure vessel. The coolant enters the reactor vessel and flows downward within the downcomer annulus created by the core barrel and the reactor vessel to the lower region of the reactor vessel below the lower support plate. The coolant then flows upward through the core to remove heat generated by the fissioning of the nuclear fuel. The heated coolant then flows out of the pressure vessel to a heat exchanger, generally referred to as a steam generator, to generate steam within a secondary loop, which may then be used to drive a turbine generator to generate electrical power. The coolant then flows back into the reactor vessel to begin the process anew.
When fission first occurs, a number of the neutrons generated have a very high energy, greater than or equal to one million electron volts (E.gtoreq.1 MeV), and are commonly referred to as fast neutrons. In order for the fission reactions to be sustained, these fast neutrons must be slowed down, a process also referred to as moderation or thermalization of fast neutrons. The coolant within the reactor vessel operates as a very good neutron moderator. Some of these neutrons may be absorbed by "poisons" within the vessel, either in the form of structural materials, fission products, control rods or the boron dissolved within the coolant In this manner, together with control rod insertion and removal, the rate of nuclear fission can be adequately controlled so that the nuclear power plant can be operated in an efficient and economical manner.
Nevertheless, some of the neutrons are not captured as part of the fission process and travel through the core to the core barrel and eventually to the reactor vessel itself. Since the continued operation of a nuclear reactor requires a steady source of neutron flux, the loss of neutrons in this manner may affect the efficiency of a nuclear power plant and the natural characteristics of the reactor vessel. Efforts have been undertaken in recent years to increase the operating efficiency of a nuclear reactor by reclaiming these neutrons. The means chosen for this effort has been to provide for a neutron reflector in the irregular area of the reactor between the core and the core barrel; that is, the area generally referred to as the baffle-barrel region. This has the added benefit of reducing radiation effects on the reactor vessel and peripheral components. Examples of this effort are U.S. Pat. No. 4,751,043 issued on June 14, 1988, entitled "Radial Neutron Reflector" and U.S. Pat. No. 4,849,162 issued on July 18, 1989, entitled "Modular Radial Neutron Reflector", both of which are assigned to the present assignee. By way of brief explanation only, these applications disclose a method of replacing most of the reactor coolant in the baffle barrel region with a neutron reflecting material, such as stainless steel or Zircaloy, to reflect these neutrons back towards the nuclear fuel within the reactor core. Typically, about 90% of this region is to be occupied by reflector material.
Although the use of reflector material in the region between the core and the core barrel can enhance the efficiency of the nuclear reactor by reflecting many of the otherwise lost neutrons back into the core to sustain the nuclear fissioning process, it is still possible for some fast neutrons to pass through the reflector and into the core barrel and reactor vessel. Typically this area is occupied by the coolant moderator medium and the various structural supports for the reactor lower internals. At the time when the lower internals were designed for their primary function of structural support, fast neutroninduced embrittlement of reactor vessel material was not as well understood as it presently is. Although the effect of fast neutron fluence, which is the flux integrated over time, on the reactor vessel is taken into account in developing its design, current lower internal designs have not been optimized to provide improved reactor vessel shielding as well as the structural functions. Current designs of lower internals do provide an amount of neutron flux reduction by the reflection or scattering of fast neutrons away from the vessel by the stainless steel components (baffle plates, formers, core barrel, and thermal shield) between the core and the vessel, and some slowing down of fast neutrons being accomplished by the water in the baffle-barrel region and in the downcomer annulus. If the energy of a significant number of fast neutrons passing through the reflector material can be reduced by interaction with an appropriate neutron shielding material, fewer neutrons will reach the reactor vessel with energies above the threshold which affects the vessel material. In this manner, the operating life of a nuclear reactor can be significantly increased.